A demand register on a watt hour meter indicates the maximum load that has been connected to the circuit since the last reading and resetting of the meter. The demand register contains a pusher arm or pusher wheel that is driven by the power measuring mechanism of the watt hour meter. The arm or wheel is periodically reset by a timing device, such as an electric clock mechanism, so that the maximum position of the arm or wheel at any time indicates the kilowatt demand during the particular interval, usually 15, 30, or 60 minutes, at which the observation is made. The demand pointer or demand wheel, driven by the pusher arm or wheel, is not restored to its initial position by the timing device but remains in the highest position to which it has been moved since the last reset by the meter reader.
At the time of reading, the position of the demand pointer or wheel is noted and the customer billed accordingly. A penalty may be imposed if the demand exceeds a predetermined amount. The maximum demand pointer or wheel is then reset for operation during the next billing period, usually one month.
Demand registers can be one of three different types. These are referred to in AEIC/EEI/NEMA Standard MSJ-4 as Single pointer Form Registers, Multiple Pointer Form Registers, and Cumulative Form Registers. The demand pointer on a Single Pointer Form Register is pushed through an arc of less than 360.degree. by its pusher arm, and the monthly reset thus requires less than 360.degree. rotation in order to return the demand pointer to zero.
A multiple Pointer Form Demand Register provides for more than 360.degree. rotation of the demand wheel and gears one or more additional demand wheels with a step-down of 10, 100, etc. This allows reading of the maximum demand on circles identical to those normally provided for reading the kilowatt hour usage. The monthly reset mechanism must also be bi-directional or provided with a slipping clutch so that a crank mechanism can return the multiple pointers to zero against a positive stop. The crank also rotates less than 360.degree. as in Ham Pat. No. 3,721,901 or Shackford 3,908,166.
A cumulative Form of Demand Register has a special purpose as will be explained later, and the actual demand is not recorded until after the monthly resetting operation has transferred the reading of a hidden demand wheel to a set of multiple pointer displays. The monthly reset mechanism for a Cumulative Form Demand Register must rotate a full 360.degree. or more so that no back-up of the mechanism is required in order to be ready for the following month.
Back-up must be prevented because of the purpose of a Cumulative Demand Register is to prevent possible collusion between the meter reader and the electricity customer. A weakness of the Single Pointer Form or Multiple Pointer Form Registers is the fact that the demand reading is destroyed at the time of monthly reset, and there is no audit means for determining if the meter reader reported the correct value. However, the reported demand reading from a Cumulative Form Demand Register can be audited any time prior to the next month's reset.
The 360.degree. or more motion is necessary so that the meter reader cannot partially reset the demand one month, back-up the reset mechanism and reseal, and then reset another portion of the demand the following month. Two or more such partial monthly resets would permit reporting of low demand readings that could not be detected by any intermediate audits.
Therefore, Cumulative Form Demand Registers have historically been provided with 360.degree. or more rotation unidirectional mechanisms such as a key-lock that must be rotated 360.degree. in one direction only before the key can be removed, or by a motorized mechanism where a push button starts a motor that cannot be interrupted, or by a spring-wound mechanism that must be cocked and triggered to prevent interruption of a complete revolution.